MOSFET Usage
in [Design]
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From: Ron M. on 25 May 2010 17:13 On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > > > > On May 22, 10:42 am, "Joe G \(Home\)" <jo...(a)optusnet.com.au> wrote: > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > >news:a67274a4-1270-45cf-8c90-4f139be8ad3c(a)y12g2000vbr.googlegroups.com.... > > > > >I am in the process of designing a device that uses 30A MOSFET for the > > > > output. I need it to apply a 12 Vdc source to a switchable load on the > > > > ground side. Are there any major issues using a MOSFET in this way? > > > > What is the load type, inductive (motor etc) capacitive, resistive? > > > > What is the peak current on switch on? > > > > Have you done thermal calcuations of the MOSFET and heat sink? > > > > What is the operating temprature range? Do the thermal calcualtions meet > > > the MOSFET thermal requirements? > > > > What is the operating voltage? Is the max_Vds of the MOSEFT much greater > > > than the operating voltage. > > > > What is the Vgs_turn_voltage? Is it much greater than the MOSFET > > > Vgs_turn_on_threshold? > > > > If you answer these questions then you are well on your way to operating the > > > MOSFET correctly. > > > > PS... good clues on MOSFET operation may be obtained from here. > > > >http://www.btipnow.com/events/training.php > > > > Joe > > > I knew this would go crazy if didn't spell things out. OK here we go. > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > plus attached all the time and the load will be between the MOSFET and > > ground and will be several loads switched in one at a time. Basically > > this is the output for a sequencer that will be used to provide power > > to several different strings of devices (9 outputs in sequence by 20 > > sequences). The different sequences will be switched in manually using > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > handle the amount of current and power factors being used. Just wasn't > > sure if the plus could be connected to the MOSFET all the time and the > > load between the MOSFET and ground. Haven't used them much and wanted > > to save myself some grief during the design stages. They aren't that > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > all the responses. Hopefully this will be enough for an informed > > opinion. THANKS folks. > > Do you want to use N-channel or P-channel mosfets. Using the latter > would be easier > for switching grounded loads.- Hide quoted text - > > - Show quoted text - The actual device is an NTE2376 30A MOSFET crossed over to an IRFP250NPbf. The device is an N-channel Enhancement type. Just want to know if I can place the load on the Source side instead of the drain side without undue stress on the component. It will be pulsed for 100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. This is a portable application for firing e-matches in fireworks in a sequential manner.
From: ian field on 25 May 2010 17:25 "Ron M." <strmbrgr2(a)hotmail.com> wrote in message news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com... On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > > > > On May 22, 10:42 am, "Joe G \(Home\)" <jo...(a)optusnet.com.au> wrote: > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > >news:a67274a4-1270-45cf-8c90-4f139be8ad3c(a)y12g2000vbr.googlegroups.com... > > > > >I am in the process of designing a device that uses 30A MOSFET for > > > >the > > > > output. I need it to apply a 12 Vdc source to a switchable load on > > > > the > > > > ground side. Are there any major issues using a MOSFET in this way? > > > > What is the load type, inductive (motor etc) capacitive, resistive? > > > > What is the peak current on switch on? > > > > Have you done thermal calcuations of the MOSFET and heat sink? > > > > What is the operating temprature range? Do the thermal calcualtions > > > meet > > > the MOSFET thermal requirements? > > > > What is the operating voltage? Is the max_Vds of the MOSEFT much > > > greater > > > than the operating voltage. > > > > What is the Vgs_turn_voltage? Is it much greater than the MOSFET > > > Vgs_turn_on_threshold? > > > > If you answer these questions then you are well on your way to > > > operating the > > > MOSFET correctly. > > > > PS... good clues on MOSFET operation may be obtained from here. > > > >http://www.btipnow.com/events/training.php > > > > Joe > > > I knew this would go crazy if didn't spell things out. OK here we go. > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > plus attached all the time and the load will be between the MOSFET and > > ground and will be several loads switched in one at a time. Basically > > this is the output for a sequencer that will be used to provide power > > to several different strings of devices (9 outputs in sequence by 20 > > sequences). The different sequences will be switched in manually using > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > handle the amount of current and power factors being used. Just wasn't > > sure if the plus could be connected to the MOSFET all the time and the > > load between the MOSFET and ground. Haven't used them much and wanted > > to save myself some grief during the design stages. They aren't that > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > all the responses. Hopefully this will be enough for an informed > > opinion. THANKS folks. > > Do you want to use N-channel or P-channel mosfets. Using the latter > would be easier > for switching grounded loads.- Hide quoted text - > > - Show quoted text - The actual device is an NTE2376 30A MOSFET crossed over to an IRFP250NPbf. The device is an N-channel Enhancement type. Just want to know if I can place the load on the Source side instead of the drain side without undue stress on the component. It will be pulsed for 100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. This is a portable application for firing e-matches in fireworks in a sequential manner. ************* Shouldn't be a problem, but if you want to ground one side of the load and still use an N-channel MOSFET, you have to devise an extra supply rail above Vdd. Look at the data sheet - there is a minimum Vgs to achieve the headline RDSon.
From: Ron M. on 25 May 2010 18:31 On May 25, 5:25 pm, "ian field" <gangprobing.al...(a)ntlworld.com> wrote: > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com... > On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > > > > > > > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > On May 22, 10:42 am, "Joe G \(Home\)" <jo...(a)optusnet.com.au> wrote: > > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > > >news:a67274a4-1270-45cf-8c90-4f139be8ad3c(a)y12g2000vbr.googlegroups.com... > > > > > >I am in the process of designing a device that uses 30A MOSFET for > > > > >the > > > > > output. I need it to apply a 12 Vdc source to a switchable load on > > > > > the > > > > > ground side. Are there any major issues using a MOSFET in this way? > > > > > What is the load type, inductive (motor etc) capacitive, resistive? > > > > > What is the peak current on switch on? > > > > > Have you done thermal calcuations of the MOSFET and heat sink? > > > > > What is the operating temprature range? Do the thermal calcualtions > > > > meet > > > > the MOSFET thermal requirements? > > > > > What is the operating voltage? Is the max_Vds of the MOSEFT much > > > > greater > > > > than the operating voltage. > > > > > What is the Vgs_turn_voltage? Is it much greater than the MOSFET > > > > Vgs_turn_on_threshold? > > > > > If you answer these questions then you are well on your way to > > > > operating the > > > > MOSFET correctly. > > > > > PS... good clues on MOSFET operation may be obtained from here. > > > > >http://www.btipnow.com/events/training.php > > > > > Joe > > > > I knew this would go crazy if didn't spell things out. OK here we go. > > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > > plus attached all the time and the load will be between the MOSFET and > > > ground and will be several loads switched in one at a time. Basically > > > this is the output for a sequencer that will be used to provide power > > > to several different strings of devices (9 outputs in sequence by 20 > > > sequences). The different sequences will be switched in manually using > > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > > handle the amount of current and power factors being used. Just wasn't > > > sure if the plus could be connected to the MOSFET all the time and the > > > load between the MOSFET and ground. Haven't used them much and wanted > > > to save myself some grief during the design stages. They aren't that > > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > > all the responses. Hopefully this will be enough for an informed > > > opinion. THANKS folks. > > > Do you want to use N-channel or P-channel mosfets. Using the latter > > would be easier > > for switching grounded loads.- Hide quoted text - > > > - Show quoted text - > > The actual device is an NTE2376 30A MOSFET crossed over to an > IRFP250NPbf. The device is an N-channel Enhancement type. Just want to > know if I can place the load on the Source side instead of the drain > side without undue stress on the component. It will be pulsed for > 100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. > This is a portable application for firing e-matches in fireworks in a > sequential manner. > > ************* > > Shouldn't be a problem, but if you want to ground one side of the load and > still use an N-channel MOSFET, you have to devise an extra supply rail above > Vdd. > > Look at the data sheet - there is a minimum Vgs to achieve the headline > RDSon.- Hide quoted text - > > - Show quoted text - I checked the data sheet and am having difficulty decyphering what you mean. The Vgs is +/- 20 volts MAX but the Gate Threshold Voltage is between 2 and 4 volts. Like I said I'm kind of new to designing with MOSFET's. I'm more of a bi-polar kind of tech. Boy!! That didn't sound right but I guess you get the gist of what I mean.
From: neddie on 26 May 2010 02:23 On May 26, 12:31 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > On May 25, 5:25 pm, "ian field" <gangprobing.al...(a)ntlworld.com> > wrote: > > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > >news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com.... > > On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > > > > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > > On May 22, 10:42 am, "Joe G \(Home\)" <jo...(a)optusnet.com.au> wrote: > > > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > > > >news:a67274a4-1270-45cf-8c90-4f139be8ad3c(a)y12g2000vbr.googlegroups..com... > > > > > > >I am in the process of designing a device that uses 30A MOSFET for > > > > > >the > > > > > > output. I need it to apply a 12 Vdc source to a switchable load on > > > > > > the > > > > > > ground side. Are there any major issues using a MOSFET in this way? > > > > > > What is the load type, inductive (motor etc) capacitive, resistive? > > > > > > What is the peak current on switch on? > > > > > > Have you done thermal calcuations of the MOSFET and heat sink? > > > > > > What is the operating temprature range? Do the thermal calcualtions > > > > > meet > > > > > the MOSFET thermal requirements? > > > > > > What is the operating voltage? Is the max_Vds of the MOSEFT much > > > > > greater > > > > > than the operating voltage. > > > > > > What is the Vgs_turn_voltage? Is it much greater than the MOSFET > > > > > Vgs_turn_on_threshold? > > > > > > If you answer these questions then you are well on your way to > > > > > operating the > > > > > MOSFET correctly. > > > > > > PS... good clues on MOSFET operation may be obtained from here. > > > > > >http://www.btipnow.com/events/training.php > > > > > > Joe > > > > > I knew this would go crazy if didn't spell things out. OK here we go. > > > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > > > plus attached all the time and the load will be between the MOSFET and > > > > ground and will be several loads switched in one at a time. Basically > > > > this is the output for a sequencer that will be used to provide power > > > > to several different strings of devices (9 outputs in sequence by 20 > > > > sequences). The different sequences will be switched in manually using > > > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > > > handle the amount of current and power factors being used. Just wasn't > > > > sure if the plus could be connected to the MOSFET all the time and the > > > > load between the MOSFET and ground. Haven't used them much and wanted > > > > to save myself some grief during the design stages. They aren't that > > > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > > > all the responses. Hopefully this will be enough for an informed > > > > opinion. THANKS folks. > > > > Do you want to use N-channel or P-channel mosfets. Using the latter > > > would be easier > > > for switching grounded loads.- Hide quoted text - > > > > - Show quoted text - > > > The actual device is an NTE2376 30A MOSFET crossed over to an > > IRFP250NPbf. The device is an N-channel Enhancement type. Just want to > > know if I can place the load on the Source side instead of the drain > > side without undue stress on the component. It will be pulsed for > > 100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. > > This is a portable application for firing e-matches in fireworks in a > > sequential manner. > > > ************* > > > Shouldn't be a problem, but if you want to ground one side of the load and > > still use an N-channel MOSFET, you have to devise an extra supply rail above > > Vdd. > > > Look at the data sheet - there is a minimum Vgs to achieve the headline > > RDSon.- Hide quoted text - > > > - Show quoted text - > > I checked the data sheet and am having difficulty decyphering what you > mean. The Vgs is +/- 20 volts MAX but the Gate Threshold Voltage is > between 2 and 4 volts. Like I said I'm kind of new to designing with > MOSFET's. I'm more of a bi-polar kind of tech. Boy!! That didn't sound > right but I guess you get the gist of what I mean. Your gate needs to driven higher than the source , by at least the threshold voltage. Look at it like this. If you turn on the gate with 5V , the fet turns on. The source voltage (voltage across the load)now climbs and the fet will start to turn off. This will continue until you can drive the gate higher than the source by the threshold voltage. Your supply to the fet is 12V , so you want to drive the gate to at least 16V(assuming 4V threshold). As mentioned in a previous post , you can use a voltage doubler to achieve the higher voltage needed to drive the gate very easely. If no "ac" supply is around , maybe you can use another 555 to do the job. I'll modify the previous cct I posted to show you how. Just jive me a few minutes. Rob
From: neddie on 26 May 2010 02:52
On May 26, 8:23 am, neddie <seegoo...(a)yahoo.com> wrote: > On May 26, 12:31 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > > On May 25, 5:25 pm, "ian field" <gangprobing.al...(a)ntlworld.com> > > wrote: > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > >news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com.... > > > On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > > > > > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > > > On May 22, 10:42 am, "Joe G \(Home\)" <jo...(a)optusnet.com.au> wrote: > > > > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > > > > >news:a67274a4-1270-45cf-8c90-4f139be8ad3c(a)y12g2000vbr.googlegroups.com... > > > > > > > >I am in the process of designing a device that uses 30A MOSFET for > > > > > > >the > > > > > > > output. I need it to apply a 12 Vdc source to a switchable load on > > > > > > > the > > > > > > > ground side. Are there any major issues using a MOSFET in this way? > > > > > > > What is the load type, inductive (motor etc) capacitive, resistive? > > > > > > > What is the peak current on switch on? > > > > > > > Have you done thermal calcuations of the MOSFET and heat sink? > > > > > > > What is the operating temprature range? Do the thermal calcualtions > > > > > > meet > > > > > > the MOSFET thermal requirements? > > > > > > > What is the operating voltage? Is the max_Vds of the MOSEFT much > > > > > > greater > > > > > > than the operating voltage. > > > > > > > What is the Vgs_turn_voltage? Is it much greater than the MOSFET > > > > > > Vgs_turn_on_threshold? > > > > > > > If you answer these questions then you are well on your way to > > > > > > operating the > > > > > > MOSFET correctly. > > > > > > > PS... good clues on MOSFET operation may be obtained from here. > > > > > > >http://www.btipnow.com/events/training.php > > > > > > > Joe > > > > > > I knew this would go crazy if didn't spell things out. OK here we go. > > > > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > > > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > > > > plus attached all the time and the load will be between the MOSFET and > > > > > ground and will be several loads switched in one at a time. Basically > > > > > this is the output for a sequencer that will be used to provide power > > > > > to several different strings of devices (9 outputs in sequence by 20 > > > > > sequences). The different sequences will be switched in manually using > > > > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > > > > handle the amount of current and power factors being used. Just wasn't > > > > > sure if the plus could be connected to the MOSFET all the time and the > > > > > load between the MOSFET and ground. Haven't used them much and wanted > > > > > to save myself some grief during the design stages. They aren't that > > > > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > > > > all the responses. Hopefully this will be enough for an informed > > > > > opinion. THANKS folks. > > > > > Do you want to use N-channel or P-channel mosfets. Using the latter > > > > would be easier > > > > for switching grounded loads.- Hide quoted text - > > > > > - Show quoted text - > > > > The actual device is an NTE2376 30A MOSFET crossed over to an > > > IRFP250NPbf. The device is an N-channel Enhancement type. Just want to > > > know if I can place the load on the Source side instead of the drain > > > side without undue stress on the component. It will be pulsed for > > > 100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. > > > This is a portable application for firing e-matches in fireworks in a > > > sequential manner. > > > > ************* > > > > Shouldn't be a problem, but if you want to ground one side of the load and > > > still use an N-channel MOSFET, you have to devise an extra supply rail above > > > Vdd. > > > > Look at the data sheet - there is a minimum Vgs to achieve the headline > > > RDSon.- Hide quoted text - > > > > - Show quoted text - > > > I checked the data sheet and am having difficulty decyphering what you > > mean. The Vgs is +/- 20 volts MAX but the Gate Threshold Voltage is > > between 2 and 4 volts. Like I said I'm kind of new to designing with > > MOSFET's. I'm more of a bi-polar kind of tech. Boy!! That didn't sound > > right but I guess you get the gist of what I mean. > > Your gate needs to driven higher than the source , by at least the > threshold voltage. > Look at it like this. If you turn on the gate with 5V , the fet turns > on. The source voltage (voltage across the load)now climbs > and the fet will start to turn off. This will continue until you can > drive the gate higher than the source by the threshold voltage. > Your supply to the fet is 12V , so you want to drive the gate to at > least 16V(assuming 4V threshold). > As mentioned in a previous post , you can use a voltage doubler to > achieve the higher voltage needed to drive the gate > very easely. If no "ac" supply is around , maybe you can use another > 555 to do the job. > I'll modify the previous cct I posted to show you how. Just jive me a > few minutes. > Rob I've modified the earlier cct to use N-channel mosfets. It's just a 555 astable driving a voltage doubler , to give you about 24V. The extra transistor is just to level shift and give you 24V drive to the gate of the FET. Note that the Vgs of the FET is not being exceeded , its still only seeing 12 across gate-source. It's not the best cct in the world , but it will work and uses a 555 which you are already using. If you have another AC source , replace 555 with that. If its is only 50 or 60 hz , you mat have to increace the cap values in the doubler cct a bit. P.S I've bees assuming you have LTSpice , if not get it from Linear.com. It's a great program and is free!! 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